This invention relates generally to producing prosthetic medical devices. More specifically, the invention provides methods to produce compositions made of polyethylene which have been irradiated in such a manner so that only a selected percentage of the overall composition has been allowed to cross-link. The capability to precisely control the degree and location of cross-linking in a polymer has particular advantages in the orthopedic device arts.
Many prosthetic medical devices are implanted into load-bearing joints such as knees, hips, etc. As such, these prosthetic devices must be very strong and possess a high degree of wear resistance. The prosthetic medical device industry has utilized various methods and compositions employing metals and polymers and combinations thereof to fabricate prosthetic devices. Prosthetic medical devices manufacturers constantly work toward developing better products by improving their physical properties. Improved wear resistance, for example, is a desirable quality to impart to a prosthetic medical device. Improving wear resistance without losing strength or causing oxidative degradation is a difficult balance to obtain.
Various methods of manufacturing compositions of polymeric materials have been devised with the objectives of reducing wear rate and improving the oxidation resistance of the polymeric materials used to fabricate prosthetic medical devices. U.S. Pat. Nos. 6,017,975, 5,879,400, 5,414,049 and 5,728,510 are referenced herein to illustrate the common methods and compositions used to fabricate polymeric prosthetic devices presently employed in the field.
One common practice within the prosthetic medical device industry is to use cross-linked polymers and resins to form the medical device. xe2x80x9cCross-linkedxe2x80x9d polymers are defined as polymeric materials which have been subjected to chemical or radiation-initiated activation resulting in dendritic bond formation between and amongst individual polymeric chains yielding new intermolecular and intramolecular networks. These cross-linked networks within the polymer provide chemical and physical properties which are usually different from the virgin polymer. Such properties include increased wear and creep resistance, durability, etc. Indiscriminate or uncontrolled cross-linking of the polymeric material comprising the prosthetic device may result in improved wear resistance, but strength and other desirable properties may be sacrificed.
Another difficulty conventionally encountered in the manufacturing process of polymeric components of ball and socket or bearing-type prosthetic medical devices, such as hips, knees, and other load-bearing joints, is that they cannot be formed easily by inexpensive injection molding techniques. Instead, these particular types of prostheses must first be formed into a stock bar or rod, by extrusion for example, after which further machining is necessary to form the finished article. Injection molding, on-the-other-hand, allows for the final article to be formed in virtually one step.
Therefore, a need exists within the prosthetic medical device industry to fabricate an improved polymeric prosthetic device possessing sufficient strength to withstand the stress and pressure imposed on it, yet resist wear. There also exists a need to fabricate the devices inexpensively by injection molding. The present invention provides compositions, as well as methods of improving the wear resistance of prosthetic medical devices, by selectively cross-linking a polymeric resin using a controlled cross-linking process providing improved strength and wear resistance.
The present invention also provides compositions and methods of injection molding and selectively cross-linking prosthetic medical devices thus rendering an inexpensive, and more facile prosthetic medical device fabrication process.
The present invention provides methods of producing selectively cross-linked polyethylene orthopedic devices. Specifically, the invention provides a localized and controlled cross-linking method used to produce orthopedic implant prosthesis having improved wear characteristics. The localized and controlled degree of cross-linking is accomplished by exposing a polyethylene object or pre-formed orthopedic prosthetic joint or limb bearing surface to an interrupted, masked or pulsed radiation source. The interrupted radiation source may be accomplished by various means, all of which limit the amount of radiation ultimately contacting the object. By interrupting or limiting the radiation exposure to certain sites on the polyethylene object or prosthetic device, cross-linking only occurs where the radiation is able to contact or penetrate the object. Other areas not so contacted with the radiation either do not become cross-linked or only peripherally so. The invention therefore, allows a technician not only to control where the cross-linking will take place within or on the surface of a workpiece, but also the degree to which the polymer ultimately becomes cross-linked. By limiting or selectively cross-linking the polymeric device, one can impart specific desirable properties to the polymer not normally present in the raw polymer or in the fully cross-linked polymer of the prior art.
It is therefore one aspect of the invention to provide a process for preparing an orthopedic device by preferably providing a polyethylene workpiece such as a stock bar or rod, or alternatively, a pre-formed joint or limb bearing. The polyethylene workpiece is then positioned in the path of a radiation beam. Preferably, a beam interrupter is placed between the workpiece and the beam source. The radiation source is then activated so that the beam is cast toward the workpiece but preferably interrupted partially by the interrupting means. The workpiece is preferably exposed to the interrupted radiation beam for a certain amount of time known to produce the desired amount of cross-linking. The degree of cross-linking imparted to the workpiece may correspond to a specific degree of mechanical toughness and wear resistance in the finished prosthesis.
The present invention provides for the fabrication of various types of prosthetic devices. While the invention is not limited to any particularly shaped prosthetic device, the preferred shapes include acetabular cups, knees, ankles, shoulders, tibial and femoral joints, finger and thumb members, vertebra, elbows, foot and toe members and wrist members.
In another aspect of the invention the polymeric materials used to form the prosthetic device may selected from the group of polyethylenes including, but not limited to, high molecular weight polyethylene (HMWPE), ultra high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), ultra high density polyethylene (UHDPE), cross-linked polyethylene and non-cross-linked polyethylene. In this aspect of the present invention, any combination of polymers listed above, or their equivalents, may be used. A preferred polymer of the invention is UHMWPE, and a preferred combination is UHMWPE and HDPE.
It is another aspect of the invention to provide a mask, shield or screen to serve as the interrupting means. The mask may preferably be a perforated sheet preferably made of metal, graphite or other thermally stable equivalent material. The number of perforations would preferably correspond to the ultimate exposure, and therefore the cross-linking of the workpiece. Another interrupting means may preferably be a wire mesh which would also limit the amount of radiation ultimately reaching the workpiece depending on the mesh size of the sheet. It is preferable to practice the present invention with individual interrupting devices, however, any combination may be used.
It is another aspect of the present invention to provide a curved interrupting means, preferably a perforated sheet or wire mesh sheet. The curved sheet or mask may partially or completely surround the workpiece but in either case provide partial shielding of the radiation beam to the workpiece, and thus serve as an interrupting means. The curved sheet or mask may be contoured to match the surface of a prepared polymeric workpiece or preformed prosthetic device such as, for example, an acetabular cup or tibial member.
Yet another aspect of the present invention is to provide a rotating chopper wheel serving to interrupt the beam intermittently, thereby introducing cross-linking to specific areas or regions of the workpiece.
Another object of the invention is to provide a pulsed radiation beam, thereby limiting the degree of radiation ultimately contacting the workpiece.
The present invention also provides a method of irradiating a workpiece using a plurality of radiation sources. The radiation sources may preferably be directed in the same or different directions, all contacting the workpiece.
It is yet another aspect of the invention to provide a method whereby the workpiece is preferably completely surrounded by an interrupting means, preferably a perforated cage, whereby a preferably plurality of radiation sources are directed to the workpiece from various directions to provide an all-encompassing and uniform radiation exposure to the workpiece.
It is yet another aspect of the invention to rotate or otherwise translate the workpiece while it is exposed to the interrupted radiation beam.
In another aspect of the invention, the workpiece and/or the interrupting means such as a perforated mask or wire mesh is vibrated while in the path of the radiation beam.
It is yet another aspect of the invention to form a prosthetic device or pre-formed article by injection molding which is suitable for ball and socket and bearing-type prosthetic joints.